This invention relates to an electromagnetic induction heater for use in heating a continuous thin sheet due to electromagnetic induction.
An electromagnetic induction heater of the type described is operable to heat by the use of electromagnetic induction a thin sheet, such as a strip, which is very thin as compared with its breadth. This electromagnetic induction heater is usually equipped with electromagnets disposed with a space left therebetween so as to induce eddy currents on the strip which is transported between the space in a predetermined direction. In order to induce eddy currents on the strip, the electromagnets are energized by an alternating current.
In the electromagnetic induction heater, it is desirable that the strip is uniformly heated while the strip is transferred within the space interposed between the electromagnetics. In addition, it is also preferable that, even if the strip is varied in breadth, uniform heating of the strip can be achieved.
To accomplish the above-described uniform heating, such a conventional heater is disclosed in U.S. Pat. No. 4,678,883, by Hajime Saitoh and Morio Maeda, assigned to Sumitomo Heavy Industries, Ltd. that is the same company as an assignee of this invention. In the conventional heater mentioned above, each of the electromagnets is constitutedby a plurality of magnetic pole-segments separately disposed in a direction transverse to the predetermined direction and common coils which wind the magnetic pole-segments. Such a heater may be called a transverse magnetic flux type of an electromagnetic induction heater. The magnetic pole-segments can be individually moved towards the strip. In addition, a shielding plate of a nonmagnetic material is disposed at each end portion of each magnetic pole-segment which is near the strip. Such a shielding plate serves to abruptly weaken a magnetic field which is generated at both breadthwise ends of the strip and which may be referred to as a fringing field.
Herein, let the strip of a ferromagnetic material be heated by the use of the conventional heater. In this event, the strip is likely to be undesirably or unevenly attracted to both the magnetic-pole segments opposed to one another. Moreover, consideration should be made about the fact that the strip of the ferromagnetic material has a Curie point and drastically changes its characteristics from one to another at the Curie point. Therefore, such a strip of a ferromagnetic material must be uniformly and accurately heated rather than the other strip.
At any rate, the conventional heater is not suitable for heating a strip of a ferromagnetic material because the strip is unevenly heated by the conventional heater. Such uneven heating brings about occurrence of a warp or undulation on the strip. In addition, when the strip is brought into contact with the magnetic-pole segments, the strip is undesirably broken off, which results in a reduction of a yield of the strip.
In order to prevent the strip from being broken off, attempts will have been made to widen a space interval of the magnetic-pole segments. However, these attempts are disadvantageous in that heat efficiency is degraded because a lot of magnetic-pole segments should be arranged over a range wider than the width of the strip.